1. Field of the Invention
The present invention relates to an artificial vessel and, in particular, to an apparatus and method for permitting long-term extracorporeal circulation of blood flow from and to the vasculature of a patient.
2. Description of the Related Art
It is often necessary to divert the flow of blood from a patient""s blood vessel back to the same or a different blood vessel as part of treating a patient suffering from one or more of numerous health impairments, including cardiovascular ills. In many cases, such efforts involve using artificial means for carrying the blood between vessels. The materials selected for doing so depend often on whether the application is acute (short-term) or chronic (long-term). In either case, it is beneficial to employ biocompatible materials, although the extent of biocompatibility differs depending upon the duration of intended or expected use with the patient. Biocompatibility is generally measured by how little the synthetic material adversely affects the patient""s blood and tissues. Materials that eventually destroy red blood cells or body tissues are generally not suitable particularly for long-term applications.
For short-term or acute applications, a wide range of polymer materials are available, such as polyethylene, silicone and polyvinyl chloride (PVC). While the level of biocompatibility for such polymer materials is not particularly high, for short-term use, the adverse effects on the patient tend to be minimal. For chronic or long-term applications of artificial blood vessels used for the diversion of blood, the need for a higher level of biocompatibility rises dramatically. Indeed, an entire industry has evolved around the development of biocompatible materials that may be formed as conduits to function as artificial vessels for carrying diverted blood to and from a patient""s vascular system on a long-term basis. Examples of such materials are ePTFE (expanded polytetrafluroethylene) such as that manufactured by Bard Impra and woven polyester such as that manufactured by W. L. Gore. Discussions of such synthetic biocompatible materials may be found in U.S. Pat. Nos. 5,718,973, 5,629,008 and 5,549,657.
In many cases, the blood being diverted remains entirely within the patient""s body; i.e., intracorporeal application, using a graft. Under those circumstances, the material chosen for long-term, purely internal, application need only withstand the conditions of a singular environmentxe2x80x94the interior of the patient. In some cases, a portion of the patient""s existing vascular system is used to divert the blood, ensuring complete biocompatibility. In other cases, synthetic materials are used for the graft, such as ePTFE or woven polyester. In one method of application, both ends of the artificial vessel are grafted directly to the patient""s blood vessels. Where the artificial vessel is applied entirely within the patient""s thorax, the vessel is often applied during open-chest surgery. In some cases, the artificial vessel is applied to blood vessels in a manner that does not require open-chest surgery. In those cases, the graft may be tunneled under the skin and surgically applied at both ends to the respective blood vessels. While common graft materials such as ePTFE and woven polyester are somewhat porous, it is not a problem as the pores in the wall of the graft eventually clot off.
Where there is a desire or need to divert the flow of blood externally to the patient for some period of time during treatment, the material selected to carry the blood should be capable of withstanding the conditions of two environments, that inside the body and that outside the body. Presently, the short-term application of diverting blood extracorporeally, such as perisurgical environments where the blood is diverted through an oxygenator outside the body, e.g., during cardiac surgery, an artificial vessel made of PVC is used to carry the blood. The connection to the patient""s vascular system is typically made, under such circumstances, with cannulas temporarily inserted into the vasculature of the patient for both the inflow and the outflow. An example of such an artificial vessel is made by Medtronic, Inc. The nature of the PVC material is such that it is not porous, so there is no risk of blood seeping through the walls of the artificial vessel or contaminants passing to the blood from the external environment.
The long-term application of diverting blood extracorporeally involves the use of a bi-material conduit, where one portion of the conduit is made of a biocompatible material, such as ePTFE, and the other portion of the catheter is made of a polymer material such as PVC. Typically, the ePTFE portion is anastamosed to the patient""s vasculature to permit fluid communication. The polymer portion of the catheter is used to connect to a pump and/or other device through which the blood passes.
There is one artificial vessel system manufactured by MEDOS AG, Germany that includes a closed end at the proximal end of the catheter, in which a small orthogonally positioned hole is provided to permit the physician to grasp the closed end with a hook. A tunneling guide is used to create a tunnel below the patient""s skin through which the cannula may reside. The guide is then used to grasp the closed end of the vascular conduit and pull the proximal end of the vascular conduit through the tunnel created by the guide. A hemostat is placed over the proximal end of the conduit to seal the inner lumen and the closed end is then sliced off, permitting the proximal end of the cannula to be connected to a pump or other device. The hemostat can then be released to permit blood flow to the pump or the other device. This system provides a means for attaching the vascular conduit to the patient at a location different than the location where the vascular conduit exits the body. Where it is desired to locate the exit site proximate the connection site, no tunneling may be necessary.
Typical graft materials such as ePTFE and woven polyester are effective at diverting blood flow without adversely affecting the properties of the blood or the characteristics of the flow. However, as alluded to above, common graft materials are porous and are not successful in applications outside the body because of fluid communication with the ambient environment. Materials such as ePTFE and woven polyester, however, are simply not capable of withstanding extracorporeal environments without adversely affecting blood flow characteristics or without contamination of the blood. Contact of the blood with air may lead to contamination and infection or may lead to the more serious event of introducing air emboli into the blood stream. Thus, the industry presently relies upon the non-porous polymer materials to carry the blood outside the body. While easy to use, the problem with such materials is that they eventually have an adverse effect on the blood during prolonged use. Moreover, such materials eventually lead to poor sustained blood flow due to resulting thrombosis within the artificial vessel. Should the thrombus break away, it could lead to blood clots in other parts of the circuit or in the patient, the well known adverse results of which include occlusion of blood vessels potentially leading to stroke or myocardial infarction. In some cases, when using such polymer materials, a heparin coating has been applied to the polymer graft to minimize thrombosis. The long-term effectiveness of such an application is not certain.
Overcoming many if not all of the limitations of the prior art, the present invention comprises an extracorporeal vascular conduit for circulating blood outside a patient""s body over an extended period of time in a manner that minimizes risk of thrombosis and inflammatory response and maximizes the ability of a patient to be ambulatory during recovery stages. The inventive vascular conduit solves the needs described above by employing a single lumen vascular conduit comprising a first biocompatible material that preferably extends the majority of the length of the cannula and a second material that surrounds the portion of the conduit that extends from outside the patient""s body to just within the patient""s body. The majority of the portion of the first material that extends within the patient""s body does not have the second material. A third interface material is applied close to the distal end of the second material of the cannula to permit a physician to more effectively secure the catheter to the patient""s skin to minimize relative movement. Such a unique arrangement provides for combining the advantage of having the blood come into contact solely with a proven biocompatible material with the advantage of using polymer materials that deal with the external environment more effectively and the advantage of immobilizing the cannula more effectively to the patient.
In one preferred embodiment, the present invention comprises an extracorporeal vascular conduit comprising a length of material, such as PTFE, which could particularly be ePTFE if so desired, having a first diameter, in which a portion of the ePTFE material proximal of the distal end is enshrouded with a thin coating of medical grade silicone or polyurethane. The distal end of the vascular conduit is configured to connect to a patient""s vascular system via, for example, an end-to-side anastomosis connection. A polyester sleeve is provided close to the distal end of the thin coating of medical grade silicone or polyurethane, positioned to correspond with the skin exit site of the patient. At the proximal end, the catheter includes a relatively short, tapered section comprising silicone or polyurethane material for connecting to a pump or other device. The length of the conduit not covered by the second material depends upon where the treating physician desires to locate the transdermal site relative to the location of the connection to the patient""s vascular system. In addition, a reinforcing member, such as a helical coil, may be provided for at least some of the length of the cannula.
In another embodiment, the distal end of the vascular conduit comprises a plurality of discrete smaller conduits each of which may be connected to the patient""s vascular system at different locations. By providing a plurality of vascular connections, a large volume of flow within the vascular conduit in fluid communication with the patient may be achieved while the size of the individual conduits engaging blood vessels is maintained relatively small. This solves, among other things, sealing problems that arise as the diameter of the vascular conduit approaches the diameter of the vessel to which it attaches. Preferably, the multiple connection conduits converge distal to the transdermal penetration site so that blood flows out of the body in one conduit. As mentioned above, the plurality of discrete conduits may be made of ePTFE.
In yet another embodiment, the vascular conduit comprises a plurality of lumens. One of the lumens may be attached to a blood vessel, for example an artery of the patient""s vascular system, while another lumen is attached to another blood vessel, for example, a vein of the patient""s vascular system or another artery. Blood may be withdrawn from one vessel and then returned to the other. For example, blood in one of the patient""s veins may be withdrawn and then recirculated to one of the patient""s arteries. Alternatively, blood may be withdrawn from one of the patient""s arteries and recirculated to one of the patient""s veins. Of course, this conduit can also be used to withdraw blood from one of the patient""s arteries and return it to a second artery; or blood may be withdrawn from one of the patient""s veins and returned to another vein.
The above described embodiments, as well as other embodiments disclosed herein, could also employ various additional coatings. For example, an anti-bacterial or anti-microbial coating may be applied to reduce infection risk; an anti-thrombotic coating may be applied to reduce adhesions to the catheter housing and any other component that comes into contact with blood for any significant period of time.
In a preferred method of use, the present invention comprises the steps of (a) providing an extracorporeal vascular conduit comprising a first synthetic biocompatible material that extends substantially the length of the conduit, a second synthetic polymer material employed over the portion of the conduit configured to extend outside the patient""s body to just within the patient""s body, and a third interface material to enhance securement of the conduit to the patient""s skin at the transdermal location, (b) securing a distal end of the extracorporeal vascular conduit to the patient""s vascular system, and (c) connecting a proximal end of the extracorporeal vascular conduit to a pump, monitor, or other device used in and/or during the treatment of a patient. The method may further comprise the step of providing an anti-bacterial or anti-microbial coating to lessen infection risk.